Stud Find is an iPhone application that uses the device's built-in magnetometer to find metal studs, screws, nails and anything metallic in a wall.
The iPhone's three-axis magnetometer is an instrument that measures magnetic forces. Internal Apple software uses the instrument to position the phone's 360° orientation. Stud Find uses the magnetometer as a metal detector. — enr.construction.com
Find it here in the app store. The reviews are mixed.